1. Technical Field
The invention relates to a color signal processing device, an image processing apparatus, a color signal processing method, an image processing method, a computer-readable medium and a computer data signal.
2. Related Art
An image forming device for forming an image with coloring materials has a coloring material reduction mode in which a used amount of coloring materials can be reduced to suppress a cost required for forming the image.
FIGS. 13A and 13B are diagrams for explaining an example of a method for reducing an amount of coloring materials according to the related art. FIGS. 13A and 13B show examples of a color reproduction gamut in a lightness (L*)-chroma (C*) plane of a certain hue in an output device. FIG. 13A shows an example in which an amount of coloring materials is reduced by changing the lightness. The lightness of each of colors shown by black circles is increased to convert these colors into colors shown by white circles. Since a bright color is generally small in consumption amount of a coloring material, the consumption amount of respective coloring materials can be reduced as a whole. However, since lightness of high-lightness colors and lightness of high-chroma colors can only be increased to a contour of the color reproduction gamut, the consumption amount of coloring materials cannot be reduced so much. Also, almost all of the high-lightness colors become white. Furthermore, there is a tendency that the colors before and after the conversion are recognized as different colors.
FIG. 13B shows an example in which an amount of coloring materials is reduced by changing the chroma. The chroma of each of colors shown by black circles is reduced to convert these colors into colors shown by white circles. Since the consumption amount of coloring materials increases with increase of chroma of a color, the consumption amount of respective coloring materials can be reduced as a whole because of the reduction of chroma. However, high-lightness colors and low-lightness colors are originally not so high in chroma, the consumption amount of coloring materials cannot be reduced so much. Also, since the conversion reduces the chroma of each color, there is a tendency that an image after the conversions is recognized as a degraded image.
Also, there is a method for uniformly reducing the consumption amounts of respective coloring materials being used. However, according to this method, the difference between the colors before and after the conversion becomes remarkable as compared with the above-described method of changing lightness or chroma.
Furthermore, there is a method for changing both of the lightness and the chroma. According to this method, the recognized degree of the difference between the colors before and after the conversion becomes small as compared with the above-described method of changing either lightness or chroma. However, colors, which are recognized as having small differences from colors output from an output device, change in a hue direction in the uniform color space such as L*a*b* or LCh. Therefore, changing both of the lightness and the chroma causes resultant colors to be recognized as being different from original colors. It is noted that the colors, which are recognized as having small differences, generally depends on characteristics of the output device.